Trench power MOSFET

ABSTRACT

A device includes a semiconductor region of a first conductivity type, a trench extending into the semiconductor region, and a conductive field plate in the trench. A first dielectric layer separates a bottom and sidewalls of the field plate from the semiconductor region. A main gate is disposed in the trench and overlapping the field plate. A second dielectric layer is disposed between and separating the main gate and the field plate from each other. A Doped Drain (DD) region of the first conductivity type is under the second dielectric layer, wherein an edge portion of the main gate overlaps the DD region. A body region includes a first portion at a same level as a portion of the main gate, and a second portion at a same level as, and contacting, the DD region, wherein the body region is of a second conductivity type opposite the first conductivity type.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No. 14/525,433, entitled “Trench Power MOSFET,” filed Oct. 28, 2014, which application is a divisional of U.S. patent application Ser. No. 13/486,681, entitled “Trench Power MOSFET,” filed on Jun. 1, 2012, now U.S. Pat. No. 8,896,060, which applications are incorporated herein by reference.

BACKGROUND

In a conventional split-gate trench power Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), a polysilicon gate is divided into an upper part and a lower part, both formed in a trench. The upper part and the lower part are separated from each other by a dielectric layer. The upper part serves as the main gate for controlling the channel of the power MOSFET, and the lower part serves as the field plate for reducing surface electrical field. Accordingly, the depth of the main gate depends on the depth of the trench and the thickness of the dielectric layer filled in the recess. Both the depth of the trench and the thickness of the dielectric layer suffer from process variations, and are difficult to control.

The power MOSFET includes a p-body, in which the channel of the power MOSFET is formed to connect a source region over the p-body and a drain region under the p-body. To ensure that an entirety of the channel can be controlled by the main gate, an n-type epitaxy layer that is under the p-body needs to have at least a portion at a same level as the main gate. Since the depth of the main gate is difficult to control, a large process window is required to ensure that the epitaxy region has at least a portion at a same level as the main gate. The large process window, however, means that the gate-to-drain overlap is also large, the gate-to-drain capacitance is in turn large, and the variation of the gate-to-drain capacitance is also large. This results in the degradation in the performance of the power MOSFET and the large variation in the performance of the power MOSFET.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1A through 1J are cross-sectional views of intermediate stages in the manufacturing of a trench power Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) in accordance with some exemplary embodiments;

FIGS. 2A and 2B are cross-sectional views of intermediate stages in the manufacturing of a trench power MOSFET in accordance with alternative embodiments; and

FIGS. 3A through 3H are cross-sectional views of intermediate stages in the manufacturing of a trench power MOSFET in accordance with yet alternative embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

A trench power Metal-oxide-Semiconductor Field Effect Transistor (MOSFET) and the method of forming the same are provided in accordance with various exemplary embodiments. The intermediate stages of forming the trench power MOSFET are illustrated. The variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIGS. 1A through 1J are cross-sectional views of intermediate stages in the formation of an n-type trench power MOSFET. Referring to FIG. 1A, semiconductor region 20, which is a portion of a semiconductor substrate, is provided. Semiconductor region 20 and the respective semiconductor substrate may have a crystalline silicon structure. Alternatively, semiconductor region 20 and the respective semiconductor substrate may be formed of other semiconductor materials such as silicon germanium. The semiconductor substrate may be a bulk substrate. In some embodiments, semiconductor region 20 is a heavily doped layer doped with an n-type impurity such as phosphorous or arsenic, for example, to an impurity concentration between about 10¹⁹/cm³ and about 10²¹/cm³. In the described embodiments, the term “heavily doped” means an impurity concentration of above about 10¹⁹/cm³. One skilled in the art will recognize, however, that “heavily doped” is a term of art that depends upon the specific device type, technology generation, minimum feature size, and the like. It is intended, therefore, that the term be interpreted in light of the technology being evaluated and not be limited to the described embodiments.

Over heavily doped semiconductor region 20, epitaxy layer 22 is formed through epitaxy, and is lightly doped with an n-type impurity. The impurity concentration of epitaxy layer 22 may be between about 10¹⁵/cm³ and about 10¹⁸/cm³. Epitaxy layer 22 may be a crystalline silicon layer, although other semiconductor material may be used. Pad oxide layer 24 and hard mask 26 are then formed over epitaxy layer 22. In some embodiments, pad oxide layer 24 is formed by thermally oxidizing a top layer of epitaxy layer 22, and hence pad oxide layer 24 comprises silicon oxide. Hard mask layer 26 may be formed of silicon nitride, for example. Hard mask layer 26 is patterned to form trench 28 therein.

Next, as shown in FIG. 1B, pad oxide layer 24 and epitaxy layer 22 are etched using the patterned hard mask layer 26 as an etching mask, so that trench 28 extends into epitaxy layer 22. A further oxidation is then performed to form oxide layer 30 on the exposed surfaces of epitaxy layer 22, which exposed surfaces are in trench 28. Oxide layer 30 includes sidewall portions and a bottom portion. Next, referring to FIG. 1C, a further hard mask layer 32 is formed on oxide layer 30. Hard mask layer 32 may be formed using a conformal deposition method, and thickness T1 of the horizontal portion is close to the thickness T2 of the vertical portion of hard mask layer 32. Hard mask layer 32 includes sidewall portions on the sidewall portions of oxide layer 30, and a bottom portion on the bottom portion of oxide layer 30. In some exemplary embodiments, thickness T2 is between about 10 nm and about 1,000 nm. It is appreciated that the dimensions recited throughout the description are merely examples, and may be changed to different values.

Referring to FIG. 1D, the bottom portion of hard mask layer 32 is removed. A tilt implantation 34 is then performed to implant an n-type impurity into epitaxy layer 22, so that N-type Doped Drain (NDD) region 36 is formed in epitaxy layer 22. The NDD region 36 is self-aligned to the hard mask layer 32. The implanted n-type impurity may include phosphorous and/or arsenic. Tilt implantation 34 may include two tilt implantations that tilt to opposite directions. NDD region 36 laterally extends beyond the edges of hard mask layer 32 by distance T3. In some exemplary embodiments, distance T3 is between about 10 nm and about 1,000 nm, although distance T3 may be greater or smaller. The n-type impurity concentration of NDD regions 36 may be between about 10¹⁵/cm³ and about 10¹⁸/cm³. Furthermore, the ratio of the n-type impurity concentration in NDD regions 36 to the n-type impurity concentration of epitaxy region 22 may be greater than about 2 orders (100 times).

Next, referring to FIG. 1E, an etching step is performed to etch the bottom portion of oxide layer 30 and the underlying portion of epitaxy layer 22, so that trench 28 extends down into epitaxy layer 22. The bottom surface 28A of trench 28 may be lower than the bottom surface 36B of NDD regions 36 in accordance with some embodiments. The etch step is performed using hard mask layer 32 as an etching mask. The etching may be substantially anisotropic, and some portions of NDD region 36 are left on opposite sides of trench 28.

FIG. 1F illustrates the deposition for forming dielectric layer 38. In some embodiments, dielectric layer 38 comprises silicon oxide, although other dielectric materials that are suitable for forming gate dielectrics may also be used. Dielectric layer 38 comprises a portion at the bottom of trench 28, and sidewall portions on the sidewalls of trench 28. Dielectric layer 38 may be formed using a conformal deposition method. After the formation of dielectric layer 38, a portion of trench 28 is not filled.

As also illustrated in FIG. 1F, after the formation of dielectric layer 38, a conductive material is filled into trench 28, and is then etched back. In the etch-back step, a portion of the conductive material in trench 28 remains not etched, and the portion of the conductive material outside trench 28 is removed. The remaining portion of the conductive material forms field plate 40. In some embodiments, field plate 40 comprises polysilicon, although other conductive materials such as metals, metal silicides, or the like, may also be used.

Referring to FIG. 1G, a further dielectric layer 42 is formed, and then etched back. Again, in the etch-back step, a portion of dielectric layer 42 in trench 28 remains, and the portion of dielectric layer 42 outside trench 28 is removed. Dielectric layer 42 is over field plate 40. Dielectric layer 42 may be formed of silicon oxide in some exemplary embodiments. Next, referring to FIG. 1H, hard mask layers 32 and 26 are removed, for example using diluted HF solution. Pad oxide layer 24, oxide layer 30, and dielectric layers 38 and 42, however, are not etched. Due to the removal of hard mask layer 32 from trench 28, trench 28 is expanded laterally, and pad oxide layer 24, oxide layer 30, and dielectric layer 42 are exposed.

Next, as also shown in FIG. 1H, an additional conductive material is filled into trench 28, and is then etched back. In the etch-back step, a portion of the conductive material in trench 28 remains, and the portion of the conductive material outside trench 28 is removed. The remaining portion of the conductive material forms main gate 44, which is separated from field plate 40 by dielectric layer 42.

In subsequent steps, as shown in FIG. 1I, an implantation is performed to form p-bodies 46 in a top layer of epitaxy layer 22. P-bodies 46 are formed on opposite sides of main gate 44. P-bodies 46 comprise a p-type impurity such as boron, indium, and/or the like. The bottoms 46A of p-bodies 46 are lower than the bottom surface 44A of main gate 44, and are higher than the bottom surfaces 36A of NDD regions 36. Accordingly, p-bodies 46 are in contact with the respective NDD regions 36. In some embodiments, p-bodies 46 have a p-type impurity concentration between about 10¹⁵/cm³ and about 10¹⁸/cm³. A further implantation is performed to form heavily doped n-type regions 48, which may have an n-type impurity concentration between about 10¹⁹/cm³ and about 10²¹/cm³, for example. P-bodies 46 include upper portions 46B that are level with main gate 44, and lower portions 46C that are level with, and contacting NDD regions 36.

Next, referring to FIG. 1J, Inter-Layer Dielectric (ILD) 50 is formed over the structure shown in FIG. 1I, and is formed over main gate 44. ILD 50 may comprise Phospho-Silicate glass (PSG), Boro-Silicate Glass (BSG), Boron-Doped Phospho-Silicate Glass (BPSG), Tetraethyl Orthosilicate (TEOS) oxide, or the like. ILD 50 may be formed as a blanket layer. Contact openings (filled with source regions 54 in FIG. 1J) are then formed by etching ILD 50 and some portions of heavily doped n-type regions 48. After the contact opening formation, the sidewalls of heavily doped n-type regions 48 are exposed, and the top surfaces of p-bodies 46 are also exposed.

Next, as also shown in FIG. 1J, an implantation is performed to dope a p-type impurity into the contact openings and into p-bodies 46, so that heavily doped p-type regions 52 are formed in the surface regions of p-bodies 46. Heavily doped p-type regions 52 act as the pickup regions of p-bodies 46. Next, a conductive material is deposited to form source region 54. Furthermore, a conductive material is deposited on heavily doped semiconductor region 20 to form drain region 56. Source region 54 and drain region 56 may also be deposited on the opposite surfaces of the respective wafer/die. In some embodiments, source region 54 and drain region 56 are formed of a metal or a metal alloy such as aluminum, copper, tungsten, nickel, and/or the like. Power MOSFET 60 is thus formed. The electrical connections to main gate 44 and field plate 40 may be formed by forming contact plugs, which contact plugs are not in the plane shown in FIG. 1J, and hence are not shown. In some embodiments, field plate 40 is electrically coupled to, and at a same voltage as, source region 54. In alternative embodiments, field plate 40 is disconnected from source region 54, and is applied with a voltage separate from the voltage of source region 54.

An on-current of power MOSFET 60 is schematically illustrated using curved lines 61, which pass through source region 54, heavily doped n-type regions 48, channel regions 46′ in p-type bodies 46, NDD regions 36, epitaxy layer 22, and semiconductor region 20, and reach drain region 56.

It is observed that main gate 44 and NDD regions 36 have overlaps, the width of the overlapped area is affected by thickness T3 of NDD regions 36. The overlap width and the overlap width variation are both small. Accordingly, since NDD regions 36 form parts of the drain region of power MOSFET 60, the gate-to-drain capacitance is small. It is observed that the overlap width is substantially not affected by the process variation in the depth of various feature formed in the steps shown in FIGS. 1E through 1H.

FIGS. 2A and 2B illustrate cross-sectional views of intermediate stages in the formation of a power MOSFET in accordance with alternative embodiments. Unless specified otherwise, the materials and formation methods of the components in these embodiments and the embodiments in FIGS. 3A through 3H are essentially the same as the like components, which are denoted by like reference numerals in the embodiments shown in FIGS. 1A through 1J. The details of the like components shown in FIGS. 2A through 3H may thus be found in the discussion of the embodiment shown in FIGS. 1A through 1J.

The initial steps of these embodiments are essentially the same as shown in FIGS. 1A through 1C. Next, as shown in FIG. 2A, an etch step is performed to remove the bottom portion of hard mask layer 32, and extend trench 28 into epitaxy layer 22. The etching may be anisotropic, and the remaining hard mask layer 32 is used as the etching mask, which covers edge portions of trench 28, and leaves the center region of trench 28 not covered. Next, as shown in FIG. 2B, NDD regions 36 are formed by tilt implantation 34. In some embodiments, to form NDD regions 36, mask region 37 may be formed at the bottom of trench 28, so that the bottom surface of NDD regions 36 is higher than the bottom of trench 28. Mask region 37 is then removed. In alternative embodiments, mask region 37 is not formed, and a proper implantation angle is used such that the bottom surface of NDD regions 36 is higher than the bottom of trench 28. The NDD region 36 is self-aligned to the hard mask layer 32. Subsequent steps in these embodiments are essentially the same as what are shown in FIGS. 1F through 1J, and are not repeated herein.

FIGS. 3A through 3H illustrate cross-sectional views of intermediate stages in the formation of a trench power MOSFET in accordance with yet alternative embodiments. The initial steps of these embodiments are essentially the same as shown in FIGS. 1A and 1B. Next, as shown in FIG. 3A, conductive layer 62 is formed. In some embodiments, conductive layer 62 is formed of polysilicon, and is referred to as polysilicon layer 62 hereinafter, although other conductive material such as metals, metal alloys, metal silicides, and the like, may also be used.

Referring to FIG. 3B, polysilicon layer 62 is etched. The horizontal portions of polysilicon layer 62 are removed, and some vertical portions of polysilicon layer 62 remain in trench 28. Next, the remaining portions of polysilicon layer 62 are used as a mask to etch oxide layer 30 and the underlying portion of epitaxy layer 22. Trench 28 thus extends into epitaxy layer 22.

Referring to FIG. 3C, NDD regions 36 are formed through the tilt implantations of an n-type impurity. In some embodiments, to form NDD regions 36, mask region 37 may be formed at the bottom of trench 28, so that the bottom surface of NDD regions 36 may be higher than the bottom of trench 28. In alternative embodiments, mask region 37 is not formed, and a proper implantation angle is used such that the bottom surface of NDD regions 36 is higher than the bottom of trench 28. The NDD region 36 is self-aligned to the hard mask layer 32. It is observed that remain portions of polysilicon layer 62 overlap NDD regions 36, wherein the overlap width is close to the smaller one of thickness T5 of polysilicon layer 62 and thickness T3 of NDD regions 36. In FIG. 3D, dielectric layer 38 is formed, followed by the formation of field plate 40 at the bottom of trench 28. The formation of field plate 40 comprises a deposition step and an etch-back step. In FIG. 3E, dielectric layer 38 is etched back, and dielectric layer 42 is formed to cover field plate 40.

Next, in FIG. 3F, dielectric layer 42 is etched back so that the vertical portions of dielectric layer 42 on the sidewalls of trench 28 and the portions of dielectric layer 42 outside trench 28 are removed, while the portion over field plate 40 is left un-etched. A conductive material is then deposited, and then etched back. The portion of the conductive material in trench 28 is merged with the remaining portion of polysilicon layer 62 to form main gate 44. In some embodiments, the conductive material comprises polysilicon. It is appreciated that visible interface 44C may exist in main gate 44 since the two portions of main gate 44 are formed at different times. FIG. 3G illustrates the removal of hard mask layer 26. FIG. 3H illustrates the formation of p-bodies 46, heavily doped n-type regions 48, ILD 50, heavily doped p-type regions 52, source region 54, and drain region 56. The process details are not repeated herein.

Although the embodiments shown in FIGS. 1A through 3H provide methods of forming n-type power MOSFETs, the teaching is readily available for the formation of p-type power MOSFETs, with the conductivity types of the respective regions 20, 22, 36, 46, 48, and 52 inverted.

In accordance with embodiments, a device includes a semiconductor region of a first conductivity type, a trench extending into the semiconductor region, and a field plate in the trench, wherein the field plate is conductive. A first dielectric layer separates a bottom and sidewalls of the field plate from the semiconductor region. A main gate is disposed in the trench and overlapping the field plate. A second dielectric layer is disposed between and separating the main gate and the field plate from each other. A DD region of the first conductivity type is under the second dielectric layer, wherein an edge portion of the main gate overlaps the DD region. A body region includes a first portion at a same level as a portion of the main gate, and a second portion at a same level as, and contacting, the DD region, wherein the body region is of a second conductivity type opposite the first conductivity type.

In accordance with other embodiments, a device includes a semiconductor region of a first conductivity type selected from the group consisting essentially of p-type and n-type, and a trench extending from a top surface of the semiconductor region into the semiconductor region. A first dielectric layer is at a bottom and sidewalls of the trench. A field plate includes a bottom and sidewalls contacting the first dielectric layer, wherein the field plate is conductive. A main gate is in the trench and overlapping the field plate. A second dielectric layer is between and separating the main gate and the field plate from each other. A DD region of the first conductivity type includes a top surface contacting the second dielectric layer, and a sidewall contacting the first dielectric layer. The DD region has an impurity concentration greater than an impurity concentration of the semiconductor region.

In accordance with yet other embodiments, a method includes epitaxially growing an epitaxy semiconductor region of a first conductivity type, forming a first trench in the epitaxy semiconductor region, and forming an implantation mask extending into the trench and covering edge portions of the trench. A tilt implantation is performed to form a DD region in the epitaxy semiconductor region, wherein the DD region includes portions overlapped by the implantation mask, and wherein the DD region is of a first conductivity type. The epitaxy semiconductor region is etched to extend into the epitaxy semiconductor region, wherein the step of etching is performed using the implantation mask as an etching mask. After the step of etching, a first dielectric layer is formed lining a bottom and sidewalls of the trench. A field plate is formed in the trench and over the first dielectric layer, wherein the field plate includes a bottom and sidewalls contacting the first dielectric layer. A second dielectric layer is formed over the field plate. A main gate is formed in the trench and over the second dielectric layer.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A method comprising: forming a trench in a semiconductor region, wherein the semiconductor region is of a first conductivity type; forming a conductive layer extending into the trench and on an edge of the trench; etching the semiconductor region to extend the trench deeper into the semiconductor region, with the conductive layer acting as an etching mask; performing a tilt implantation using the conductive layer as a part of an implantation mask to form a Doped Drain (DD) region in the semiconductor region, wherein the DD region is of the first conductivity type; forming a first dielectric layer on a bottom and sidewalls of the trench; forming a field plate in the trench and over a bottom portion of the first dielectric layer; forming a second dielectric layer over the field plate; and forming a main gate in the trench and over the second dielectric layer.
 2. The method of claim 1, wherein the forming the conductive layer comprises: depositing a blanket conductive layer comprising horizontal portions and vertical portions, wherein the trench has a remaining portion unfilled by the blanket conductive layer; and etching the blanket conductive layer to remove the horizontal portions, wherein parts of the vertical portions in the trench remain as the conductive layer.
 3. The method of claim 2, wherein a top end of the conductive layer is lower than a top surface of the semiconductor region.
 4. The method of claim 1, wherein the DD region is overlapped by the conductive layer.
 5. The method of claim 1, wherein the main gate is joined to the conductive layer to form a continuous conductive region.
 6. The method of claim 1 further comprising forming a mask layer at the bottom of the trench, wherein a topmost end of the mask layer is lower than a top of the trench, with the tilt implantation performed using the mask layer as an additional implantation mask, and the DD region has a bottom surface higher than the bottom of the trench.
 7. The method of claim 1 further comprising: depositing an inter-layer dielectric over the main gate; and forming a source region and a drain region on opposite sides of the semiconductor region.
 8. The method of claim 1 further comprising forming a body region of a second conductivity type opposite to the first conductivity type, wherein the body region contacts a sidewall of the DD region.
 9. A method comprising: etching a semiconductor region to form a trench; forming a mask layer at a bottom of the trench; performing a tilt implantation to form a Doped Drain (DD) region in the semiconductor region, with the tilt implantation performed using the mask layer as an additional implantation mask, wherein a topmost surface of the mask layer defines a bottom surface of the DD region, wherein the DD region has portions on opposite sides of the trench, and a bottom surface of the DD region is higher than a bottom of the trench; forming a first dielectric layer lining the bottom and sidewalls of the trench, with the first dielectric layer having an edge contacting an edge of the DD region; forming a field plate in the trench and over a bottom portion of the first dielectric layer; forming a second dielectric layer over the field plate; and forming a main gate in the trench and over the second dielectric layer.
 10. The method of claim 9, wherein the trench comprises a portion that is overlapped by the DD region.
 11. The method of claim 9, wherein the tilt implantation is performed with a conductive layer in the trench as an implantation mask.
 12. The method of claim 11 further comprising: depositing a blanket conductive layer comprising horizontal portions and vertical portions; and etching the blanket conductive layer to remove the horizontal portions, wherein parts of the vertical portions in the trench remain as the conductive layer.
 13. The method of claim 9, wherein a bottom surface of the first dielectric layer is in physical contact with a top surface of the semiconductor region.
 14. The method of claim 9 further comprising removing the mask layer after the tilt implantation.
 15. The method of claim 9, wherein the top surface of the mask layer is between a top of the trench and the bottom of the trench.
 16. A method comprising: etching a semiconductor region to form a trench; oxidizing a surface layer of the semiconductor region to form an oxide layer, wherein the surface layer is exposed to the trench; forming a conductive layer in the trench and on sidewalls of the trench, wherein the conductive layer has a bottom contacting a top surface of the oxide layer; etching the oxide layer and the semiconductor region to extend the trench deeper into the semiconductor region, with the conductive layer acting as an etching mask; forming a mask layer extending to a bottom of the trench, wherein the mask layer has a topmost surface at an intermediate level between a top surface of the semiconductor region and the bottom of the trench; performing a tilt implantation to form a Doped Drain (DD) region in the semiconductor region, with the tilt implantation performed using the mask layer as an implantation mask, wherein the topmost surface defines a position of a bottom surface of the DD region, wherein the DD region is overlapped by the conductive layer; implanting the semiconductor region to form a body region, with the body region and the DD region having opposite conductivity types, wherein the body region has a bottom end at an intermediate level between a top surface and a bottom surface of the DD region, and the body region contacts a sidewall of the DD region; and forming a drain region overlapped by the DD region, wherein the drain region is electrically connected to the DD region.
 17. The method of claim 16, wherein when the tilt implantation is performed, the conductive layer has a top surface lower than a top surface of the semiconductor region.
 18. The method of claim 16, wherein when the tilt implantation is performed, a bottom surface of the conductive layer is higher than a bottom of the trench.
 19. The method of claim 16 further comprising: forming a first dielectric layer on a bottom and sidewalls of the trench, with the first dielectric layer having an edge contacting an edge of the DD region; forming a field plate in the trench and over a bottom portion of the first dielectric layer; forming a second dielectric layer over the field plate; and forming a main gate in the trench and over the second dielectric layer.
 20. The method of claim 16, wherein the bottom surface of the DD region is substantially coplanar with the topmost surface of the mask layer. 